In many high-power systems, the combination of an ultracapacitor and a battery works much better than the battery by itself. But present-day systems, even those that employ both an ultracapacitor and a battery, leave much room for improvement. The system performance is not nearly as good over the full range of actual loads and service use. And the battery does not last as long as one might wish. As will be explained in great detail below, the invention offers better performance and an expectation of longer battery life. But first, some background will be provided which may establish some terminology and which may help in an appreciation of the great needs that were until now unfulfilled.
FIG. 1 shows a simplified block diagram of a hybrid automobile 21. An internal combustion engine and alternator 23 provide electric power to motor-generator 24, which is in turn coupled mechanically to a drive train and wheels 25. Importantly, the automobile uses regenerative braking, in which the automobile is slowed down by converting kinetic energy through the drive train 25 to the motor-generator 24, into electrical energy which is stored in energy storage system 26. Energy storage system 26 is idealized as a two-terminal device at terminals 30, 31. In many present-day hybrid cars the chief (or perhaps only) component of the energy storage system 26 is an electrochemical battery 22. Energy developed through regenerative braking and stored in storage system 26 may later be used instead of, or in addition to, that of the engine-alternator 23 to power the motor-generator 24.
As compared with a conventional automobile using only an internal combustion engine, the hybrid automobile 21 offers improved fuel economy, in part because of the use of regenerative braking. The improved fuel economy is gained at a cost, of course, namely the not insubstantial manufacturing cost of the battery 22 as well as a later disposal or recycling cost of that very large battery. Remarkably, even though the automobile 21 is burdened with having to move a very heavy battery 22 from place to place, the automobile 21 nonetheless enjoys the improved fuel economy just mentioned.
Although hybrid cars are still relatively recent in day-to-day experience, it is already beginning to be appreciated that battery life matters a lot. If some invention were to offer the prospect of a greater battery life, this would be very good news for at least three reasons—first, the substantial expense of the battery in the first place could be spread out over a longer service life, second, the expense of disposal or recycling of the battery could be postponed, and third, the inconvenience to the user of battery failure or the related out-of-service interval during battery replacement could be reduced or postponed.
As mentioned above, FIG. 1 is simplified. For example FIG. 1 shows a single motor-generator 24 connected by means of a transfer case and two differentials to the four wheels. It should be appreciated, however, that another approach is to employ four motor-generators, each coupled mechanically to a respective wheel.
It will also be appreciated that the automobile 21 has switches and other control electronics that are omitted for clarity in FIG. 1, but that are important to bring about the same results as an accelerator pedal or brake pedal in a conventional car. There are also fuses or fusible links in the lines connecting to the battery 22, so as to cut power in the event of a short circuit external to the battery 22. There may also be one or more temperature sensors within the battery 22 which are intended to permit derating the battery until the over-temp condition has passed.
FIG. 2 shows a typical energy storage system 26 such as is shown in FIG. 1, in greater detail. Battery 22 contains a number of electrochemical cells 27 in series. (It will be appreciated that series-parallel arrangements, though not shown in FIG. 2, may also be employed.)
The battery 22 is an electrochemical device, storing and releasing energy through chemical reactions. The chemical reactions cannot happen instantly but take some time. The chemical reactions can also generate some heating. This means that the battery cannot deliver all or part of its stored energy instantly, but takes some time to deliver its energy. This likewise means that if energy enters the storage system from outside (for example due to regenerative braking), such energy cannot be instantly stored to the battery but takes some time to do so. Efforts to draw large amounts of energy very quickly from the battery can lead to heating which offers several drawbacks—first, the heating represents a waste of energy that could otherwise have been put to some good use, second, the heating represents a need to design a mechanism for dissipating the heat from the battery, and third, the heating can reduce the service life and can increase the risk of premature failure.
For a given choice of battery chemistry, and for a given detailed battery design, experience shows that a battery will have only some limited number of charge-discharge cycles available in its service life. (As mentioned above, operational extremes such as over-temp or overly high current drains or charging currents could reduce the number of available charge-discharge cycles from what would otherwise be available.)
These factors and others have, in very recent times, prompted system designers to try putting an ultracapacitor in parallel with the battery. FIG. 3, for example, shows an energy storage system 26 composed of an ultracapacitor 28 (composed of a number of individual ultracapacitors 29 in series) in parallel with a battery 22. This approach does offer benefits over the use of the battery 22 alone (as in FIG. 2). The ultracapacitor can receive energy (for example from regenerative braking) very quickly, much faster than a battery can. When energy is to be delivered to a load (for example for quick acceleration), the ultracapacitor can supply energy to that load very quickly, again faster than a battery can.
As mentioned above, experience shows that the combination of an ultracapacitor and a battery often works much better than the battery alone. This has prompted investigators to try using larger and smaller ultracapacitors to see whether there is (for a particular application) some optimum ratio of size for the ultracapacitor and the battery. This has also prompted investigators to consider whether a more complicated connection between the ultracapacitor, the battery, and the load may lead to better results than the simple connection portrayed in FIG. 3.
One approach is to make use of a “power converter” in connection with the ultracapacitor and the battery. In a typical embodiment, a power converter forces current to flow in either of two directions, as directed by a control line indicative of desired current flow. Such a power converter may, for example, contain two buck converters, one pointed in each direction, and two boost converters, one pointed in each direction, and control electronics turning on one of the four converters (to the exclusion of the other three) with whatever duty cycle is required to bring about the desired current flow. In a null state (no control signal applied) the converter will preferably come as close as possible to being a straight wire.
FIG. 4 shows an energy storage system 34 which presents itself to the load as a two-terminal device at pins 30 and 31. Ultracapacitor 28 may be seen, along with battery 20. Importantly, in the system of FIG. 4, there is a power converter 32 which presents itself to the system as a two-terminal device at pins 35 and 36. A control line 33 delivers control signals to the power converter 32.
FIG. 5 shows an energy storage system 34 in an alternative to that of FIG. 4. The system 34 in FIG. 5 again presents itself to the load as a two-terminal device at pins 30 and 31. Ultracapacitor 28 may again be seen, along with battery 20. Importantly, in the system of FIG. 5, the power converter 32 is in series with the battery 20 rather than being in series with the ultracapacitor 28. Again a control line 33 delivers control signals to the power converter 32.
Turning momentarily ahead to FIG. 10, we see a more detailed depiction of the storage system 34 of FIG. 4. Terminals 30, 31 connect to loads and/or power sources external to the system 34. A battery 22 connects to the terminals 30, 31 through current sensor 49 and voltage at the battery 22 is measured by voltage sensor 57. Capacitor 28 is provided, which is preferably an ultracapacitor. Power converter 32 permits controlled passage of power from the battery to the capacitor or from the capacitor to the battery (or not at all) under control of current drive line 33. Voltage sensor 47 senses the voltage across the capacitor 28, and current sensor 45 senses current into or out of capacitor 28. Current sensor 53 senses current into or out of battery 22. Temperature sensors 43, 51 sense internal temperature of capacitor 28 and battery 22 respectively. Sense lines 44, 46, 48, 52, 54, 50, 58 provide information from the just-described sensors to an energy management system controller 55. Controller 55 provides a current drive signal at the current drive line 33. Controller 55 has a bidirectional control/data bus 56 which may be communicatively coupled with circuitry external to the architecture 34. Other signals, omitted for clarity in FIG. 10, can include a signal from a sensor of ambient temperature in the system, and a “status” signal from the power converter.
In FIG. 10 the energy management system (EMS) controller 55 monitors the ultracapacitor temperature, current and voltage (at lines 44, 46, and 48 respectively), the battery temperature, current and voltage (at lines 52, 54, and 58 respectively), and the current to and from the load (at line 58). The controller 55 has control inputs from CAN or other communications channel 56, and reports out to the same communications channel 56, and generates the power converter current command on line 33.
Turning back now to FIG. 6 we see an exemplary embodiment, with the energy storage system 34 (from FIG. 4) visible, coupled with motor-generator 24, which is in turn mechanically coupled with drive train 25.
It is known to employ a controller such as depicted in FIGS. 10 and 4 to attempt to optimize the performance of the system 34. For example such a controller may have a set-point such that efforts are made to push the charge on the ultracapacitor toward a level that is 77 percent of capacity. It turns out, however, that with typical real-life drive trains and real-life loads, the ability of the ultracapacitor 28 to provide its benefits will quickly collapse as the capacitor drains to near zero (at a time when the capacitor is being asked to supply power to the load, probably with the power converter in boost mode toward the load). Likewise when the ultracapacitor 28 is nearly full, then its benefits will quickly disappear if there is a need for the ultracapacitor to absorb a lot of energy from regenerative braking.
It would be desirable if approaches could be found that would permit better and more sophisticated control of the energy storage system. If, for example, the controller could be configured more intelligently, this might bring about the desired improved control of the energy storage system. Such approaches might offer better battery life, might reduce losses due to heating, might permit the designer to get by with a smaller ultracapacitor, and might permit the designer to get by with a smaller battery.